The U.S. Environmental Protection Agency is refusing to disclose records about a new class of pesticides that could be playing a role in the disappearance of millions of honeybees in the United States, a lawsuit filed Monday charges.

The Natural Resources Defense Council wants to see the studies that the EPA required when it approved a pesticide made by Bayer CropScience five years ago.

The environmental group filed the suit as part of an effort to find out how diligently the EPA is protecting honeybees from dangerous pesticides, said Aaron Colangelo, a lawyer for the group in Washington.

In the last two years, beekeepers have reported unexplained losses of hives - 30 percent and upward - leading to a phenomenon called colony collapse disorder. Scientists believe that the decline in bees is linked to an onslaught of pesticides, mites, parasites and viruses, as well as a loss of habitat and food.$15 billion in crops

Bees pollinate about one-third of the human diet, $15 billion worth of U.S. crops, including almonds in California, blueberries in Maine, cucumbers in North Carolina and 85 other commercial crops, according to the U.S. Department of Agriculture. Not finding a cause of the collapse could prove costly, scientists warn.

Representatives of the EPA said they hadn't seen the suit and couldn't comment.

Clothianidin is the pesticide at the center of controversy. It is used to coat corn, sugar beet and sorghum seeds and is part of a class of pesticides called neonicotinoids. The pesticide was blamed for bee deaths in France and Germany, which also is dealing with a colony collapse. Those two countries have suspended its use until further study. An EPA fact sheet from 2003 says clothianidin has the potential for toxic chronic exposure to honey bees, as well as other pollinators, through residues in nectar and pollen.